1. Field Of The Invention
This invention relates to optical communication, and, more particularly, to optical printing with a beam subject to extreme power variation.
2. Brief Description Of The Background
Optical communication is of benefit in many applications, and is a necessity for providing feedback for synchronizing optical printing systems of high quality. Such printing systems utilize a laser light source to form a light beam which is modulated by data to be printed and is swept in a raster pattern to selectively discharge a statically charged photoconductor, forming a latent charge image. The latent image is dusted with toner, which is transferred to a sheet of paper and then affixed to the paper.
It is important in printing to have exact alignment of adjacent scan lines so that printing transverse to the scans appears continuous and smooth. Any misalignment becomes highly apparent, degrading the print quality.
The scanning motion of the light beam is usually effected by a rotating mirror, and the data modulation of the beam is controlled by a precision gating device. The two devices must be accurately synchronized throughout each scan.
An example of such a printer is described in U.S. Pat. No. 3,835,249, A. Dattilo and D. Zegafuse Jr., issued Sept. 10, 1974 filed Dec. 26, 1972, Ser. No. 317,976, "Scanning Light Synchronization System," assigned in common with the present application. In that printer, synchronization is accomplished by supplying feedback signal indicative of the beam position to a shift register which precisely gates the printing data. The feedback is taken from the modulated laser writing beam as it scans using a beam splitter. Specifically, an acousto-optic modulator responds to the print data by deflecting the laser beam. Both the deflected and undeflected beams are given a scanning motion by a rotating mirror and are directed to a beam-splitting mirror. A portion of the beams (about 75%) continue through the mirror to a knife-edge slit where the normal (undeflected) beam is blocked and the deflected beam continues on to erase the static charge on a photoconductor. The reflected portion of the beam (about 25%) traverse a position optical grating system to be modulated thereby and provide the position feedback to a detector.
A similar system is employed for the IBM 6670 Information Distributor where the beam-splitting mirror is replaced by a wedge prism located prior to the acousto-optic modulator. This arrangement is described in IBM Technical Disclosure Bulletin, Vol. 22, No. 8B, D. Grant and D. Stafford, "Laser Printhead," pages 3585-6, Jan. 1980. The prism directs about 60% of the laser beam to the modulator at the Bragg angle to be selectively deflected thereby in accordance with the print data, and directs about 20% of the beam to the modulator at a different angle so that it is not selectively deflected. As before, the writing beam is scanned and either blocked by a knife-edge or erases the charge on the photoconductor. The split beam is then scanned and used in the synchronizing system, but the split beam is not at the Bragg angle and is not affected by the selective deflection of the modulator.
In each of these systems, only a portion of the laser beam ultimately becomes the writing beam, 60% in the prism system and 75% in the beam-splitting mirror system. Therefore, the power, the cost, and the power consumption of the laser are all greater than those required for the writing beam alone.